Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technique are widely deployed all over the world. High speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with highly competitive radio access technique in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future. Decrease of cost per bit, increase in service availability, flexible use of frequency bands, simple architecture and open interface, low power consumption of a user equipment and the like are suggested as requirements of next generation communication systems.
Generally, one or more cells are allocated to a base station. A plurality of user equipments can be placed in a cell. A user equipment generally performs random access procedure before accessing to a network. The objectives of the random access procedure may include 1) initial access, 2) handover, 3) scheduling request (request for radio resources), 4) timing synchronization, and the like.
Random access procedure in the initial access is performed as follows. A user equipment is turned on and tries to initially access a network. The user equipment establishes downlink synchronization with the network and receives system information from the network. Since the user equipment currently does not establish timing synchronization and does not have any uplink padio resources, the user equipment performs the random access procedure. The user equipment requests uplink radio resources for transmitting an access request message through the random access procedure. The network which receives the request for the uplink radio resources allocates the uplink radio resources to the user equipment. The user equipment can transmit the access request message to the network through the uplink radio resources.
The random access procedure can be classified into a contention based random access procedure and a non-contention based random access procedure. The most outstanding difference between the contention based random access procedure and the non-contention based random access procedure is whether a signature used for a random access preamble is dedicated to a user equipment. In the non-contention based random access procedure, since a user equipment uses a dedicated signature assigned only to itself as the random access preamble, a collision with other user equipments do not occur. The collision means that two or more user equipments simultaneously try a random access procedure using the same random access preamble through the same radio resources. In the contention based random access procedure, since a user equipment arbitrarily selects one signature among shared signatures as the random access preamble, the collision may occur.
Random access procedure in a wireless communication system based on conventional WCDMA system is described below.
The random access procedure in WCDMA system uses a physical random access channel (PRACH) as an uplink physical channel and an acquisition indicator channel (AICH) as a downlink physical channel. For the structures of the PRACH and the AICH, sections 5.2.2.1 and 5.3.3.7 of 3GPP TS 25.211 V7.0.0 (2006 March) “Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD) (Release 7)” can be referenced.
The PRACH can be divided into a preamble part and a message part. The preamble part carries a signature. At first, a physical layer of a user equipment selects an access slot and the signature. The signature is transmitted through the access slot as a random access preamble. As an example of signatures, section 4.3.3.3 of 3GPP TS 25.213 V7.0.0 (2006 March) “Technical Specification Group Radio Access Network; Spreading and modulation (FDD) (Release 7)” can be referenced. The signature is selected from sixteen signatures. The random access preamble can be transmitted for 1.33 ms duration of the access slot.
If the user equipment transmits the random access preamble to a network, the network transmits a response message to the user equipment through the AICH. The response message includes the signature of the random access preamble so that the user equipment can identify the response message as the user equipment's. The network transmits an acknowledgement (ACK) or a negative-acknowledgement (NACK) to the user equipment through the response message. If the user equipment receives the ACK from the network, the user equipment transmits the message part using an orthogonal variable spreading factor (OVSF) code corresponding to the signature. If the user equipment receives the NACK from the network, the user equipment transmits another random access preamble through the PRACH after a certain time period. If the user equipment does not receive the response message corresponding to the random access preamble, the user equipment transmits a new random access preamble with power whose level is higher by one step than that of the previous random access preamble.
The network transmits the response message as a response to the random access preamble from the user equipment. The response message includes the signature that is the same as the signature used for the random access preamble. The user equipment identifies that the response message is the response to its random access preamble through the signature of the response message.
If the response message in the random access procedure carries a variety of information as much as possible, the efficiency of the random access procedure can be improved. For example, if the response message is generated by an upper layer of the physical layer, a variety of information can be transmitted at a time through the response message.
Since the random access procedure is used for a variety of purposes, it is required that the random access procedure should be efficiently configured to enhance performance of the wireless communication system as a whole. Therefore, there is a need for a method that can more efficiently process the random access procedure.